1. Field of the Invention
The present invention relates to a cathode-ray tube used in television receivers, computer displays, and the like.
2. Related Art
Generally, color cathode-ray tube devices raster scan a phosphor screen formed on the inside of a face panel by deflecting three electron beams emitted from an in-line electron gun in horizontal and vertical directions using a deflection yoke having horizontal and vertical deflection coils.
Since the vertical deflection coil deflects the electron beams vertically, the distribution of a magnetic field generated by this coil (hereinafter, “vertical deflection magnetic field”) has a so-called barrel shape, for example. On the other hand, since the horizontal deflection coil deflects the electron beams horizontally, the distribution of a magnetic field generated by this coil (hereinafter, “horizontal deflection magnetic field”) has a so-called pincushion shape, for example. The deflection coils are made to generate magnetic fields having the above distributions in order to realize self-convergence in which the three electron beams are focused at a single point on the phosphor screen.
The vertical deflection coil is wound toroidally around a bell-shaped ferrite core attached to a resin frame (insulator). During the manufacturing process, the ferrite core sometimes gets attached with a central axis of the core offset laterally in relation to a central axis of the resin frame.
Here, “laterally” (left/right) is used to indicate the horizontal direction, with the boundary between left and right defined by a vertical plane that includes the tube axis of the cathode-ray tube when viewed from the screen side. On the other hand, “perpendicularly” (up/down) is used to indicate the vertical direction, with the boundary between up and down defined by a horizontal plane that includes the tube axis when viewed from the screen side.
When the deflection yoke is mounted in the cathode-ray tube so that the central axis of the resin frame is aligned with the tube axis of the cathode-ray tube, with the central axis of the ferrite core offset as described above, the distribution of the vertical deflection magnetic field ends up being asymmetrical on the left and right sides of the tube axis (a so-called “off-axis magnetic field”) because of a central axis of the vertical deflection magnetic field being offset from the tube axis. As a result, misconvergence occurs in which the landing position of the three electron beams is vertically displaced (hereinafter, “Yv misconvergence”).
Yv misconvergence is described here using a specific example. FIG. 1, in which a ferrite core 91 is seen from above, shows the distribution of the magnetic field generated by a vertical deflection coil when a central axis A of ferrite core 91 is offset to the right of a tube axis Z when viewed from the screen side. FIG. 2 shows Yv misconvergence that occurs on the screen when the distribution of the magnetic field generated by the vertical deflection coil is as shown in FIG. 1.
Note that a horizontal deflection coil wound around ferrite core 91 is not depicted in FIG. 1 because of a vertical deflection magnetic field 92 distributed within ferrite core 91 being displayed. The orientation of magnetic field 92 when the electron beams have been upwardly deflected is indicated in FIG. 1 by the arrows.
The electron beams, when viewed from the screen side, are emitted from an electron gun 94 arranged in-line in the order blue (B), green (G) and red (R) from the left-hand side (in FIG. 1, red, green, blue are indicated by “R”, “G”, “B”, respectively). Also, the magnetic flux density of the distribution of magnetic field 92 increases as the distance from the central axis of magnetic field 92 increases, due to the barrel shape of this magnetic field.
Consequently, the blue electron beam B positioned on the far left of the three electron beams is effected the most by the off-axis magnetic field, followed by the green electron beam G. As a result, the landing position of the three electron beams is, as shown in FIG. 2, displaced perpendicularly, being blue, green, red from the top (“R”, “G”, “B” also being used in FIG. 2 to indicate red, green, blue). Note that Yv misconvergence causes color shifts and the like.
One method for correcting Yv misconvergence involves making use of a leakage magnetic field 93 (see FIG. 1) that leaks from the magnetic field generated by the vertical deflection coil to the outside of the deflection yoke on the electron gun side (e.g. see Japanese Patent Application Publication 5-244614). According to this method, the leakage magnetic field from the vertical deflection coil is either focused or dispersed to locally strengthen or weaken the leaked magnetic field, by attaching two substantially “L” shaped magnetic bodies to the left and right of the tube axis on the end face of the deflection yoke on the electron gun side, so that the arm of each magnetic body is parallel with the tube axis.
While misconvergence can be reduced with this method, the reduction is small and the applicable range is limited.